The characteristics of a postulated cholinergic neural system involved in control of cerebral circulation will be investigated in rabbits through study of the following: 1) relationships between cortical PO2, pH and blood flow with respect to alterations in cortical electrical activity and to the effects of cholinomimetic and cholinolytic agents; 2) changes of cerebral O2 consumption in association with the cholinergically mediated cerebral blood flow changes induced by brainstem stimulation. Studies on spinal cord blood flow will be continued with emphasis on a) characterization of fiber types involved in orthodromic vasodilatation and b) determination of local tissue PO2 during anti- and orthodromic stimulation.